1. Technical Field
The present invention relates to the removal of cationic, anionic, complexed or particulate contaminants from liquids and gases. In particular, certain embodiments relate to particle clarification and sorptive-filtration by media which removes chemical constituents and particulates in a liquid or gas passed through the media.
2. Background Art
An area of increasing concern in the environmental sciences and engineering, particularly process control is the treatment or control of species that represent an environmental, ecological, human or process concern. Common examples include metal species and phases such as for metal elements Cd, Cu, Zn, Ni, Pb, As, Ag, V, and Cr, as well as non-metal species and phases such as for constituents phosphorus and nitrogen species which become water borne and are carried by urban and rural rainfall-runoff and snow-snowmelt (herein identified as “runoff”) to drainage systems (herein identified as “drainage”) receiving waters, water supplies, and to natural and anthropogenic terrestrial interfaces such as soils, the subsurface or earthen deposits. As used herein, ions or complexes being “water borne” means being transported in water in any manner, whether the ionic or complexed form is in solution, as a precipitate material, or is transported by water through a particulate bond or physical-chemical or biological attachment to a particle, in the form of a surface complex or a colloidal bond, or carried by the advective or diffusive transfer of water. Common manners in which such species and phases become water borne is through leaching, dissolution or particulate-bound entrainment by runoff from surfaces of the built or constructed environment, for example paved surfaces, or from human activities such as industry, manufacturing and agriculture. These species and phases are typically deposited on urban surfaces such as constructed surface, open surfaces, soil surfaces, and paved surfaces though vehicle exhaust, fluid leakage, vehicular wear, pavement degradation, particulate deposition, litter, illicit discharges, downspout discharges and pavement maintenance. These species and phases are typically deposited on earthen or soil surfaces through agricultural processes such as fertilization, pesticide application, insecticide application, and soil amending and land-disturbing practices such as earthwork, grading, cut/fill excavation and surficial as well as deep soil modifications. Subsequent hydrologic precipitation results in the mass transfer of these species or phases either in ionic, complexed or particulate-bound forms and transports these species and phases in surface or subsurface flows by advective, diffusive, gravitational, chemical or electromagnetic gradients.
The particulate and colloidal matter (herein identified as “particulates) itself can be deleterious, representing an environmental, ecological, human or process concern that requires control. In this application, “particulate-bound” means any bond, precipitate or complex associated with particulate material that ranges in size from colloidal (<1 μm) to suspended (1-˜25 μm) to settleable (˜25-˜75 μm) to sediment (˜75 to 4750 μm) larger gross solids or debris (>75 μm) or floatable material. While the size limits of each class of particulate matter are approximate because of properties such as specific gravity and geometry, taken in total these classes represent the entire size gradation found in surface runoff, drainage or subsurface flow. The ionic fractions can be quite variable. For example, metals such as Zn in certain source area urban watershed locations under conditions of acid rain can be greater than 80% dissolved (fd=0.8); while in other watershed or in lower locations of the same watershed, the fd for Zn can be as low as 0.2. The remaining percentage is largely particulate-bound but may be a complexed aqueous species. However, in the simplest two-phase model if the dissolved fraction is 0.8 then the particulate fraction for Zn is 0.2. This 0.2 will then distribute across the particulate size gradation as a function particle indices such as surface charge, surface area, mass and number gradation, composition of particle and contact time.
It is desirable to intercept the runoff or drainage and remove these species, phases or particulates prior to allowing the water to continue to drainage areas, water supply areas, through the subsurface or in a down-gradient transport to a sea or ocean. One method of separating the water borne species whether in dissolved ionic, complexed, precipitate or particulate-bound forms is to pass the water through a media or medium that functions to provide a range of mechanisms from surface complexation, ion exchange, adsorption, absorption or precipitation (herein collectively; identified as “sorption”) and also provides a range of mechanisms such as interception, sedimentation, impaction, straining, adhesion or physical-chemical-biological sorption of particulate matter (herein collectively identified as “filtration”). Such a media or medium is identified as providing sorptive-filtration.
One of the most common media for removing particulate bound metals from water is sand and sometimes perlite. However, sand has very little capacity for removal of dissolved or complexed species and therefore, is generally not considered effective in removing these species. A common media used for drinking water is granular activated carbon (GAC) and has long used as a media for removing dissolved organic species and also been used for species such as metals. However, for many cationic species GAC has relatively little sorptive capacity and rapid breakthrough occurs and thus, sorbed metals must frequently be removed or the GAC “recharged.” Also, GAC has low compressive strength and cannot support vertical, lateral or shear loads. Any application which places such loads on the GAC material may cause crushing, significant deformation and a greatly reduce sorptive-filtration capacity and impair physical characteristics of the GAC and the sorptive-filtration system. Similarly earthen materials such as natural perlite or modified perlite have been used for filtration and/or sorption. However perlite itself also has lower strength and loading characteristics and lower sorptive capacity for many metals and non-metals such as phosphorus.
A much more recently developed sorbent media is iron oxide coated sand (IOCS). IOCS is formed by coating silica sand with a thin layer of iron oxide and it has been shown to be an effective sorbent media for cationic species such as metals or anionic species such as phosphorus, in part dependent on the pH and point of zero charge (pzc) of the surface coating. Iron oxides and hydroxides possess little or no permanent surface charge, but will take on a positive or negative surface charge in the presence of protons or hydroxyl ions. In other words, depending on the pH of the solution in which the iron oxide is place, the iron oxide may take on a net positive or negative charge. A substance which exhibits a net positive or negative charge depending on the pH level may be referred to as an “amphoteric” substance, Iron oxide typically has a smaller net charge (either positive or negative) in a pH range of approximately 7 to 8. When the pH rises above approximately 8, the iron oxide becomes more negatively charged. Thus, positively charged cations will engage in a sorption reaction with the iron oxide surface or suspended/colloidal particulates with or without bound metal or non-metal species and borne by water passing over the negatively charged iron oxide will tend to bond to the iron oxide and be filtered from the water. Conversely, if the pH falls below approximately 7, the iron oxide becomes positively charged and is less likely to bond with cationic species, but will bond with anionic species or complexes. The pH at which the net surface charge of a particle is zero is denominated the point of zero charge or “pzc”.
One major disadvantage of IOCS, coated on an unprepared substrate surface is that the oxide coating is not sufficiently durable. For example, the comparatively smooth surface of sand particles tends to result in the oxide coating flaking off. Attempts to avoid this flaking have led to time consuming sand preparation efforts such as cleaning the sand of organics or weak surface coatings and applying a scratch surface to the sand before applying the oxide coating. However, even with these preparation efforts, IOCS still exhibits flaking and thus a reduction in oxide coating durability. The smooth surface of the sand is also disadvantageous from the standpoint of providing a comparatively low specific surface area (SSA) for bonding. The specific surface area of a material is generally defined as the surface area per unit mass with the typical unit being m2/gm. As used herein, specific surface area means the total area on the surface of the material in addition to any available porous internal surface area (such as for the GAC discussed above). The greater the surface area of the substrate, the greater the surface area of oxide coating that will be exposed to water borne metals. Thus, it is desirable to provide a substrate with a relatively large SSA not withstanding other design constraints. For example the SSA of rounded silica sand is approximately 0.05 to 0.1 m2/gm.
Another problem found with IOCS is the tendency of the oxide coating to crystallize. When the coating crystallizes, the crystals set up a morphology which does not result in the highest surface area of the coating. The surface area of the coating is much more optimal if the oxide molecules are randomly distributed in a non-lattice or “amorphous” fashion. For example, the SSA of IOCS may approach 85 m2/gm if a method of sufficiently inhibiting crystallization could be provided.